I have received a notification that my food has arrived but I do not see the driver outside my building or I can't find it at the pickup hub. 

This could be a misestimate of driver arrival time and most probably driver is a couple of minutes away. This usually happens as notifications are usually sent to you when driver is about to arrive. In some cases, a driver get's stuck in traffic very close to your location. Please give it a couple minutes after you receive the notification and if driver/food is still no show, please reach out to our support team.